1. Field of the Invention
The present invention relates to an electronically commutated direct current (d.c.) motor comprising a permanent-magnetic rotor consisting of a dome-shaped body of magnetizable material and of a permanent magnet disposed on the inner circumference thereof, and further comprising a stator provided with a winding producing an alternating current field, with the stator having a double-T-shaped design, and with the stator winding being disposed on the common leg of the double T.
2. Description of the Prior Art
Such types of d.c. motors are known (for example from German Patent Applications DE-AS Nos. 22 25 442 and 26 12 464).
These motors are single-stranded coil type two-pulse current supplied, commutatorless and permanently energized d.c. motors having an external rotor. Upon supplying the stator winding with current, these d.c. motors are capable of being restarted automatically from the standstill if only a small braking torque exists at the motor shaft. The supply of current to the stator winding is effected in these types of d.c. motors in dependence upon the rotational position of the rotor. The rotor position is ascertained by a sensor, such as a Hall generator, with a signal obtained from the sensor utilized to control an electronic circuit which applies a source of direct current voltage to the stator winding for forming the required starting torque.
These types of d.c. motors all have one drawback which finds its expression in the difficulties of generating a starting torque. In these d.c. motors, with the aid of the electronically controlled stator winding, there is produced a stator alternating current field which, in cooperation with the exciting field of the permanent magnet, forms a pulsating moment. In cases where both the polar axis of the permanent magnet and the polar axis of the alternating current field are not displaced in relation to one another, the torque equals zero. Under these conditions the rotor is prevented from starting on its own.
The forming of a starting torque requires an angle differing from zero to exist between the polar axis of the permanent magnet and the polar axis of the stator alternating current field. This means to imply that in the case of a currentless stator winding, the polar axis of the permanent magnet must be displaced by a certain angle of rotation with respect to the polar axis of the stator alternating current field.
This necessary displacement of the rotor with respect to the stator in the rest position of the motor is realized in the conventional type of d.c. motor by an asymmetrical air gap between the stator and the rotor, which is variable in the circumferential direction. The magnetic resistance of the air gap, which is not constant along the circumference of the motor, results in the permanently excited rotor, owing to the asymmetry of the air gap, being displaced with respect to the polar axis of the stator by an angular amount. The possibility of optimizing this angle by the corresponding design of the air gap is restricted because this action simultaneously reduces the utilization of the permanent magnet.
The stator of the conventional type of d.c. motor is made from a one-piece sheet steel lamination. The stator winding is wound onto the stator lamination stack as composed of this sheet steel lamination. This type of stator winding, however, is costly and involves a considerable investment. Moreover, it is not suitable for use in automatic production.